1. Field of the Invention
This invention relates generally to electron discharge device control systems, and is concerned more particularly with a cathode ray type tube connected to control circuitry having means for maintaining cathode current at a predetermined level for a predetermined operating condition of the tube.
2. Discussion of the Prior Art.
An electron discharge device of the cathode ray type generally comprises a tubular envelope having therein an electron gun disposed to direct an electron beam onto a spaced anode target electrode. The anode target may include an imaging screen layer of phosphor material which fluoresces locally when a discrete region thereof is penetrated by beamed electrons. Consequently, the electron gun may be provided with focusing means for directing the beamed electrons onto a discrete area of the imaging screen layer. Also, the tube may be provided with electron deflection means for causing the beam to scan over successive discrete areas of the imaging screen layer in a conventional line-by-line raster pattern. Thus, by suitably modulating the intensity of the scanning electron beam, there may be produced on the imaging screen layer a corresponding visible light image, which may be viewed visually through an adjacent transparent faceplate portion of the envelope.
The electron gun usually comprises an axially aligned series of spaced electrodes including an electrically heated cathode, an apertured control grid, and one or more beam forming electrodes which focus electrons passed through the control grid onto a spot area of the imaging screen layer. Generally, the spot area of the the imaging screen layer is maintained as small as possible in order to provide maximum resolution in the image. However during life test when the tube electrodes are maintained at respective constant potentials, it may be found that the cathode current of the beam has decreased from an initial value. An attempt to return the cathode current to the initial value by making the potential of the control grid less negative results in an increase in the size of the spot produced on the imaging screen layer, thereby degrading resolution.
Therefore, it is advantageous and desirable to provide a cathode ray type tube with external circuitry having means for maintaining cathode current substantially constant for a predetermined control grid bias potential without increasing the size of the spot area on the imaging screen layer.